Polyester resins represented by polyethylene terephthalate resins (hereinafter abbreviated as “PET resin” in some cases) has characteristics such as excellent heat resistance, weather resistance, mechanical properties, and transparency. Making use of such characteristics, polyester resins have widely been used not only for fiber or magnetic tapes, but also for preforms used to produce beverage containers, injection molded articles used for various purposes, or extrusion molded articles such as wrapping films or sheets. In particular, a hollow body produced by blow molding of a preform has excellent characteristics in terms of light weight, impact resistance, transparency, and the like. Accordingly, such hollow bodies have increasingly been used for containers for various types of beverages such as carbonated drinks, juice, tea, or mineral water, or containers for liquid condiments of foods such as soy sauce, sauce, salad oil, cosmetics, or liquid detergent. It is expected that the market will further expand in the future. It is required that such containers not affect the taste of the contents thereof, as well as having excellent strength, impact resistance, and transparency. Thus, it is required that polyester resins used for the aforementioned purposes be of high quality such that it has a high polymerization degree, is not colored, and contains a very small amount of impurities such as acetaldehyde. In addition, it is strongly desired that such polyester resins be able to be produced in an industrially stable manner and with good productivity at a low cost.
As a method of producing polyester resins used for the aforementioned purposes, a lower alcohol diester of PTA such as terephthalic acid (hereinafter abbreviated as “PTA”) or dimethyl terephthalate (hereinafter abbreviated as “DMT”) and alkylene glycol such as ethylene glycol (hereinafter abbreviated as “EG”) are subjected to a transesterification or direct esterification in the absence or presence of a catalyst such as a metal carboxylate, so as to produce an intermediate such as bis-β-hydroxyethyl terephthalate (hereinafter abbreviated as “BHET”) or its oligomer in advance. Thereafter, the above intermediate or oligomer, which is in a molten state, is heated under reduced pressure in the presence of a polycondensation reaction catalyst. While alkylene glycol generated as a by-product is discharged from the reaction system, melt polymerization is then carried out until the desired polymerization degree is achieved, so as to produce polyester resins.
Alternatively, a polymer pellet with a medium polymerization degree is produced by the above described melt polymerization, and it is then heated in a solid state, under reduced pressure, or in an inert gas current. Thereafter, solid phase polymerization is carried out by discharging alkylene glycol generated as a by-product from the reaction system for high polymerization, so as to produce polyester resins (see e.g. Patent Document 1).
In order to obtain a molded article by solid phase polymerization, however, a polymer pellet with a medium polymerization degree, which is solidified by cooling after melt polymerization, is heated again to a high temperature, and then dried, and crystallized. Thereafter, it is subjected to solid phase polymerization for a long period of time, so as to obtain a pellet with a high polymerization degree. Thereafter, the obtained pellet is cooled again, transported, and conserved. It is then heated and dried again to supply to a melt molding machine, so that a final molded article, or a preform used to produce a container, is molded. Thus, complex processes are required for production of polyester resins.
Although such complex processes have been required, solid phase polymerization has conventionally been carried out. That is because a low polymerization temperature results in the low likelihood of a pyrolysis reaction, and coloration or decomposition products are thereby hardly generated. Moreover, since volatile impurities are volatilized and eliminated from a polymer during polymerization, a high-quality polymer can be produced. However, this technique is problematic in that it requires special and complex equipment or methods as well as a long period of time. Moreover, the technique is also problematic in that it requires an enormous amount of energy for repeating heating and cooling so many times. Furthermore, such solid phase polymerization is also problematic in that a large amount of powder polymer that is hardly melted is generated during the polymerization, and in that the thus generated polymer plays a role in foreign matter, which might inhibit molding or might deteriorate the quality of a molded article, such as in terms of surface properties, resistance, or transparency. Still further, the core portion and the surface portion of the pellets have different molecular weights, and therefore the molecular weight distribution is wide. Therefore, a change in the molecular weight before and after processing is so significant that it is difficult to stabilize product quality. The surface portion of the pellets has a high polymerization degree and an extremely high crystallinity. Therefore, a large amount of shear heat is generated during molding processing, and a decrease in polymerization degree due to cutting of molecular branches, the amount of acetaldehyde produced as a byproduct and the degree of coloring due to thermal degradation are significant.
To date, an attempt to obtain polyester resins with a high polymerization degree only by melt polymerization without performing solid phase polymerization has also been carried out. Since an equilibrium constant is very small in the polycondensation reaction of polyester resins, a polymerization degree can be increased only after eliminating alkylene glycol generated as a by-product from the reaction system. However, since high polymerization brings on a high viscosity, it becomes more difficult to eliminate alkylene glycol. Thus, there has been a technique of using a horizontal agitator, which enables surface renewal of a large and sufficient surface area of a reaction solution in a final polycondensation reaction vessel that causes a high polymerization degree (see e.g. Patent Documents 2 and 3). Using such a technique, polyester resins with a high polymerization degree can be obtained, but a technique of using a polymerization apparatus having a rotary drive portion in the main body thereof, such as a horizontal agitator, has the following disadvantages.
When polymerization is carried out in a high vacuum, since the rotary drive portion cannot be completely sealed, inflow of a trace amount of air cannot be prevented, and polymer coloration thereby becomes inevitable. Even when a sealing solution is used to prevent such inflow of air, mixing of the sealing solution is inevitable, and thus, the quality of a polymer is inevitably deteriorated. Moreover, even when high sealing properties are kept at the beginning of the operations, the sealing properties might be decreased during long-term operations. Thus, there is also a serious problem regarding maintenance.
Furthermore, it is also difficult to reduce the content of impurities such as acetaldehyde, which is emphasized especially in the field of beverage containers. That is because acetaldehyde is likely to be generated as a by-product due to inflow of the air, and also because since an industrial-scale apparatus, including a horizontal agitator, causes a great depth of liquid, impurities such as acetaldehyde remain in a polymer.
With regard to acetaldehyde, a technique of compulsively removing acetaldehyde from PET obtained by melt polymerization by a melt deaeration treatment or the like, and directly molding a preform in a molten state, has been recently proposed.
For example, a thermoplastic polyester obtained by melt polymerization is subjected to a deaeration treatment without substantial increase in an intrinsic viscosity, so as to decrease the concentration of acetaldehyde, and thereafter, a preform is molded (see Patent Document 4). In this technique, however, since an extruder with a vent is used in deaeration, a polyester with a high polymerization degree has an excessively high viscosity, and acetaldehyde cannot be sufficiently reduced. In addition, a polymer locally has a high temperature due to heating by shearing or a heater, strong coloration occurs due to inflow of the air from an axial sealing portion, as stated above, or a large amount of decomposition products are generated. A technique of adding a phosphate-containing compound to prevent coloration has also been proposed, but it cannot sufficiently enhance quality.
Moreover, there has been another technique whereby inert gas is injected into a polyester molten body with an intrinsic viscosity between 0.5 and 0.75 dl/g, and melt polymerization is then carried out in a polymerization reactor at a temperature between 260° C. and 285° C. under reduced pressure, so as to form a polyester molten body containing low acetaldehyde with an intrinsic viscosity between 0.75 and 0.95 dl/g, followed by injection molding of the obtained polyester molten body (see Patent Document 5). However, according to the studies of the present inventors, since a horizontal biaxial agitator-type reactor is used as a polymerization reactor in this technique, a long period of time is required for high polymerization. Further, inflow of the air from the axial sealing portion causes significant coloration. In addition, since an industrial-scale reactor causes a great depth of liquid, high polymerization is further difficult, and it also becomes impossible to reduce acetaldehyde. It is also extremely difficult to uniformly inject into a polyester molten body inert gas in an amount sufficient for deaeration in a horizontal reactor on an industrial scale.
Furthermore, there has been another technique whereby a polyester polymerized in a reactor is supplied to a mixer without solidifying it at midpoint, acetaldehyde-eliminating agents such as nitrogen or carbon monoxide are then injected into the mixer, acetaldehyde is then eliminated in a flash tank, and the residue is then transported to a molding machine, so as to obtain a molded article (see Patent Document 6). In this technique, a polyester into which a stripping agent is mixed is converted into a large number of strands, filaments, or ribbons through a die, and the thus obtained products are extruded into a flash tank in a reduced-pressure atmosphere. The thus extruded product is allowed to fall onto the bottom of the flash tank, and then it is allowed to intensively foam, so as to eliminate acetaldehyde. Regarding this technique, the form of the polymerization reactor is not described in detail. However, if a common horizontal double axis agitator-type reactor was used in this technique, a long period of time would be required for high polymerization, and further, inflow of the air from an axial sealing portion would cause significant coloration. Further, since this technique requires special auxiliary equipment such as a mixer or flash tank as well as a reactor, the processes become complicated. Furthermore, since such a mixer or flash tank has a space where a polymer can remain for a long time, pyrolysis locally progresses, and a depleted polymer that is significantly colored is mixed into a product.
Still further, there has been another technique of transporting a resin in a molten state from a polymerization machine to a molding machine and then molding it (see Patent Document 7). However, a horizontal agitating polymerization machine is used in this method. Therefore, long-term polymerization is required to achieve a high polymerization degree, and inflow of the air from an axial sealing portion causes significant coloration. A devolatilizer for eliminating acetaldehyde, etc. is essential in this method, but a polymer is required to remain in such a devolatilizer for a further period of time, so that the product is deteriorated in color and that the production cost also increases.
Still further, there has been another technique of adding an acetaldehyde scavenger as well as a devolatilizer for eliminating acetaldehyde (see Patent Document 8). However, the use of a large amount of such an acetaldehyde scavenger causes problems such as generation of odor and coloration derived from the scavenger.
As stated above, the conventional melt polymerization techniques can reduce volatile impurities such as acetaldehyde, but they cannot achieve a molded article of polyester resins, which has a high polymerization degree and a good hue.
Other than the above described technique of using a polymerization apparatus comprising a rotary drive portion in the main body thereof, a method of performing polymerization while allowing a prepolymer to fall by gravitation from the upper part of a polymerization reactor, so as to produce PET with a high polymerization degree by melt polymerization, has also been proposed from a long time ago.
For example, there has been a technique whereby filamentary polyester is allowed to fall into a vacuum space, so as to produce a polyester with a desired molecular weight (see Patent Document 9). In this technique, since recirculation of the fallen polymer results in deterioration of the quality of the produced polyester, polymerization is completed by one-pass operation. However, since it is extremely difficult to keep a sufficient polymerization time by such a method, it is also extremely difficult to obtain a polymer with a high polymerization degree. In addition, filaments are easily cut off in a polymerization reactor. This is problematic in that the quality of the obtained polymer is drastically fluctuated; and in that condensates with a low molecular weight scattered from the filaments contaminate the nozzle surface, and it becomes difficult for the filaments to be injected directly below from the nozzle due to such contamination, and as a result, the filaments come into contact with one another and are cut off, or they are gathered to become a thick filament and it is then fallen, so that it prevents the reaction.
In order to solve such inconveniences, as a continuous polycondensation method of BHET as an initial condensate of PET and/or an initial condensate as an oligomer thereof, there has been proposed a method involving polymerizing the above materials at a reactor temperature of 340° C., while allowing the materials to fall by gravitation along a linear object that is vertically hung from a nozzle in an atmosphere where inert gas is circulated (see Patent Document 10). However, according to the studies of the present inventors, EG generated as a by-product cannot be eliminated from the reaction product at a sufficient rate in such an atmosphere where inert gas is circulated. Thus, a polymer with a high polymerization degree required for beverage containers cannot be obtained. Furthermore, pyrolysis significantly occurs at a high temperature such as 340° C., and only a polymer that is colored so as to become yellow can be obtained.
In addition to the above described methods, as a method of producing a polyester and a polyamide, there has also been a method of performing polymerization while allowing a polymer to fall by gravitation along a linear support vertically disposed in a reactor (see Patent Document 11). Moreover, as a method of producing a polyester, there has also been a technique whereby a PET oligomer with a mean degree of polymerization between 8 and 12 (which corresponds to an intrinsic viscosity of 0.1 dl/g or less) is supplied at 285° C., the oligomer is allowed to fall by gravitation along a cylindrical wire gauze vertically disposed in a reactor, and at the same time, polymerization is carried out under reduced pressure in the reactor (see Patent Document 12). Furthermore, there has also been proposed a method and an apparatus of allowing a PET prepolymer with a melting viscosity of 0.5 Pa·s (which corresponds to an intrinsic viscosity of 0.1 dl/g or less) to absorb inert gas, allowing the prepolymer to fall by gravitation along a guide under reduced pressure, and at the same time, performing polymerization (see Patent Document 13).
However, according to the studies of the present inventors, a polymer with a polymerization degree of interest cannot be obtained by directly applying the above described method in industrial-scale equipment. Moreover, a polymer discharged from a perforated plate or the like intensively foams, and it contaminates the wall of the reactor provided with the support and the nozzle surface. Such contaminants are decomposed, modified, or colored during long-term operations, and these degradation products are mixed into a polymer, so that the quality of a product deteriorates.
Other than these methods, there has also been proposed a polymerization method wherein the temperature of a reaction product is continuously decreased as the reaction product falls when bis-hydroxyethyl terephthalate or an oligomer thereof is supplied to a wetted-wall column followed by continuous polymerization under reduced pressure, and at the same time, vacuum aspiration is carried out from the lower portion of the column (see Patent Document 14). However, according to the studies of the present inventors, a polymer with a high polymerization degree cannot be obtained by applying the above method. When the amount of a polymerization intermediate (prepolymer) supplied is decreased to improve a polymerization velocity, a drift (biased flow) of the polymerization intermediate (prepolymer) occurs, and thus, a high-quality polymer cannot be obtained.
Hence, the conventional gravity falling-type melt polymerization techniques (Patent Documents 9 to 14) could not provide a method of industrially stably producing high-quality polyester resins having a high polymerization degree with good productivity, which can be substituted for the solid phase polymerization technique. In addition, these gravity falling-type melt polymerization techniques give no suggestion regarding a technique of obtaining a molded article containing small quantities of low molecular weight volatile substances such as acetaldehyde.    [Patent Document 1] JP-A-58-45228    [Patent Document 2] JP-A-48-102894    [Patent Document 3] JP-A-9-77857    [Patent Document 4] JP-A-2000-117819    [Patent Document 5] Japanese Patent No. 3345250    [Patent Document 6] National Publication of International Patent Application No. 2001-516389    [Patent Document 7] National Publication of International Patent Application No. 2000-506199    [Patent Document 8] National Publication of International Patent Application No. 2002-514239    [Patent Document 9] U.S. Pat. No. 3,110,547    [Patent Document 10] JP-B-4-58806    [Patent Document 11] JP-A-53-17569    [Patent Document 12] JP-B-48-8355    [Patent Document 13] International Publication WO99/65970 pamphlet    [Patent Document 14] JP-A-58-96627
It is an object of the present invention to provide a polyester resin (particularly polyethylene terephthalate resin) having a polymerization degree ranging from a low polymerization degree to a high polymerization degree, having a reduced amount of carboxyl group on the polymer terminal and a reduced content of impurities such as acetaldehyde, generating a reduced amount of aldehyde during processing, having a favorable hue, having a narrow molecular weight distribution and having high quality and excellent moldability; a polyester resin having a reduced crystallinity and suffering a less degradation in quality during processing in addition to the above-mentioned excellent characteristics; a polyester resin having a reduced content of cyclic trimer and having excellent moldability; a polyester resin pellets having a reduced amount of fine powder, having a good handling characteristic and giving a molded article having good quality; and a preform and a hollow body formed by molding the high-quality polyester resin described above.